The present invention is directed to integrated circuits. More particularly, the invention provides a system and method for controlling frequency variations. Merely by way of example, the invention has been applied to a power converter. But it would be recognized that the invention has a much broader range of applicability.
Power converters are widely used for consumer electronics such as portable devices. The power converters can convert electric power from one form to another form. As an example, the electric power is transformed from alternate current (AC) to direct current (DC), from DC to AC, from AC to AC, or from DC to DC. Additionally, the power converters can convert the electric power from one voltage level to another voltage level. The power converters include linear converters and switch-mode converters.
The switch-mode converters often need to meet certain requirements on electromagnetic interference (EMI). A single switching frequency and its harmonics can cause EMI problems. To suppress EMI, the switching frequency is often varied by clock jittering for conventional switch-mode power converters.
For example, the clock jittering is generated by an analog circuit for frequency variations. The analog circuit provides a frequency variation signal at a frequency much lower than the switching frequency of the power converter. For example, the switching frequency is in the tens of kilo-Hz range, and the frequency of the frequency variation signal is even lower. Accordingly, the analog circuit often needs a large capacitor area and is expensive to implement.
In another example, the clock jittering is generated by a digital circuit. The digital circuit can use a counter to generate a frequency variation signal. But the frequency variation signal often has a fixed pattern without scrambling. The energy spreading may not be uniform in the frequency band of interest. In yet another example, the frequency variation is controlled by an external signal such as a power supply voltage of an integrated circuit (IC) system. But the range of frequency variation is often limited. Additionally, change of the switching frequency with the power supply voltage often makes the switch-mode power converter difficult to design.
Hence it is highly desirable to improve techniques for controlling frequency variations.